Protective boots are used to essentially isolate members, including reciprocating members such as shafts, from dirt, dust and other contaminants. To be effective, the boot must be sealed. However, if it is sealed, the internal volume of the boot will change as the shaft is reciprocated causing a force on the shaft which may adversely affect the operation, as where the shaft is a part of a cable control system. It has been common to vent the bellows or boot so that excess pressure is vented to the atmosphere. In such a construction, upon contraction of the volume, contaminants may be drawn into the boot or bellows through the vent. It is thus desirable for the protective boot to provide an essentially constant volume and constant pressure chamber around the shaft to prevent the inspiration of atmospheric contaminants into the area around the shaft.
U.S. Pat. No. 4,058,018 describes an internal pressure compensating flexible bellows having first and second longitudinal sections, joined together end to end, and adapted to envelop the connection between reciprocating elements which are moved toward and away from one another. The first section has a plurality of circumferential convolutions, or "accordion-like" folds. The second section has a series of flutes which extend longitudinally. As the convoluted section contracts the fluted section expands radially outward, that is, the entire wall of the second section expands radially or stretches, in a manner similar to a balloon being inflated. Such a construction will cause a progressive increase in pressure which may adversely affect the system by creating an excessive force resisting contraction. In addition, this radial stretching may possibly lead to premature failure of the bellows because of the repeated cycles of stretching and contraction. As described, such a construction is adapted to function from an unstretched, neutral condition to a stretched condition. As a result, the neutral position must correspond to the condition of minimum axial extension of the system.
U.S. Pat. No. 4,407,517 generally describes a protective boot for use with a solenoid. The boot moves between two ends, one sealed to a stationary housing and the other sealed to the end of a movable shaft. The boot defines a generally constant volume chamber which changes its shape longitudinally or in the same direction as the movement of the shaft. The boot itself is displaced in the longitudinal direction and it folds over the end of the shaft a its shape is changed to maintain a constant volume. This folding over of the boot may interfere with needed clearance at the end of the shaft.
It is desirable to provide a protective boot which will accommodate changes in pressure in either direction from a neutral position. It is also desirable to have a protective boot which does not change its shape longitudinally so as to fold over the end of the shaft. It is also desirable to have a protective boot which does not stretch, resulting in fatigue and possible failure.
Among the objectives of the invention are to provide a boot which functions by flexing rather than stretching; which is operable between a neutral position to a position of maximum extension in one direction and between a neutral position and a position of minimum extension in the other direction.
In accordance with the invention, the protective boot has at least one section comprising some portions which flex outwardly or pop out from a neutral position in response to a predetermined increase in pressure in the section, and other portions flex inwardly or pop in, in response to a predetermined decrease in pressure from a neutral position.
Preferably, the portions are constructed and arranged to flex outward at a predetermined pressure which is higher than the neutral or atmospheric pressure. The outward flexing or popping out of the portions occurs at the predetermined pressure and increases the volume such as to relieve the pressure. The predetermined pressure is adequate to initiate the flexing but yet is not so high as to adversely affect the reciprocation of the movable shafts.
Correspondingly, when a pressure decrease from neutral occurs, a relatively low predetermined pressure is reached which is sufficient to initiate the inward flexing from a neutral position of certain portions of the section. The relatively low pressure is not so low as to cause an inspiration of atmospheric contaminants into the boot.
The flexing action may be thought of as being similar to the flexing of containers such as thin walled oil cans. For example, the pressure in the can decreases as the fluid therein is dispensed, at a given low pressure, the walls of the can are flexed inward, thereby defining a smaller volume. Just as in the case of an oil can, the flexible portions of the invention do not respond proportionally to a pressure change, rather, the flexing action is triggered at predetermined, desired pressure. Therefore, the contour of the boot changes at predetermined pressures. The boot defines a chamber, which comprises two sections, one section of the boot comprises the flexing portion.
The protective boot comprises basically two sections, a longitudinally extendible and contractible first section and a radially extendible and contractible second section. The first section has one end adapted to be sealed to a reciprocating shaft and the other end is connected to an integral collar which provides communication between the first section and the second section. The first section has a plurality of circumferential pleats which extend circumferentially and which are spaced apart. When the shaft is extended the pleats of the first section are expanded from a neutral position. When the shaft is moved in the opposite direction from a neutral position, the pleats of the first section are spaced closer together.
The second section comprises a plurality of circumferentially spaced portions extending axially from the center of the second section, some of said portions being concave outwardly from the center and some of said portions extending axially and being concave inwardly in the neutral position connected by relatively thicker outwardly curved portions.
In the preferred embodiment there are basically three portions, relatively thicker outwardly curved portions, relatively thinner outwardly concave portions and relatively thinner inwardly concave portions. Preferably, there are four relatively thicker outwardly curved portions, and each is arranged adjacent and between a relatively thinner inwardly concave portion and a relatively thinner outwardly concave portion. Preferably, the relatively thicker outwardly curved portions are arranged in pairs, each pair being diametrically opposed at opposite ends of an axis. Preferably, there are two relatively thinner outwardly concave portions which are also diametrically opposed and there are two relatively thinner inwardly concave portions also diametrically opposed. Thus, in the preferred embodiment, there are eight portions arranged around the circumference of the second section as follows, beginning with a relatively thin outwardly concave portions: thin outwardly concave portion, thick outwardly curved portion, thin inwardly concave portion, thick outwardly curved portion, thin outwardly concave portion, thick outwardly curved portion, thin inwardly concave portion and thick outwardly curved portion. The relatively thicker outwardly curved portions are relatively rigid. The relatively thinner inwardly concave portions and the relatively thinner outwardly concave portions, are able to flex.
As the shaft is extended from a neutral position, the outwardly concave portions flex inwardly. As the shaft is retracted from a neutral position, the inwardly concave portions flex radially outwardly.
The second section is adapted to be sealed to a housing which encompasses the shaft. The second section is also connected, at its other end, to the integral collar which provides communication between the first section and the second section so that an exchange of gaseous fluid can take place between the two sections. The second section is constructed and arranged to be fixed in the longitudinal direction and it does not move longitudinally with the shaft as the shaft extends and retracts.